(i) Field of the Invention
The present invention relates to a method for preparing molded articles of an ultra-high molecular weight polyethylene which has a high melt viscosity, an excellent wear resistance and impact resistance, and a small friction coefficient.
(ii) Description of the Prior Art
Since an ultra-high molecular weight polyethylene (hereinafter referred to as UHMWPE) is excellent in wear resistance and has a small friction coefficient to a variety of materials, its application to sliding parts is now desired. For example, when an artificial joint is made from a metal or a ceramic, it cannot be used without additional treatment because of its great friction coefficient between sliding surfaces. Therefore, it is necessary that a lubricating material be interposed between the sliding surfaces.
Some properties which the lubricating material must possess are (1) to be nontoxic and odorless, (2) to provide a sufficient lubricating function over a long period of time, (3) to render the surfaces of parts smooth and to permit smooth movement, and (4) to bring about no chemical reaction. Taking into account these conditions, it will be easily understood that an ultra-high molecular weight polyethylene, UHMWPE, is suitable for lubrication.
The above mentioned lubricating material may be used in the form of plane parts on occasion, but it may usually be taken in hemispherical and more intricate forms.
Further, the UHMWPE material is eligible for use in a transfer pipe for a slurry and a powder which cause severe abrasion on metallic materials, an elbow in which the flow direction of a fluid to be transferred therethrough changes, an agitated vessel, a sliding part and the like. In addition, since the UHMWPE is excellent in impact resistance, particularly the resistance to repeated impact, it is strongly desired that the UHMWPE material be utilized as an impact-absorbing stopper, or a hollow molded article such as a bellows-shaped article or the like in various industrial machines.
Articles, sheets and plates of the UHMWPE are formed by various methods, for example, sintering, ram extrusion using a plunger pump, forging and compression molding. However, it is very difficult to directly produce thin sheets or films (hereinafter referred to simply as "sheets") of less than 3 mm in thickness from the crude powder of UHMWPE. According to present industrial practice, thin sheets of this kind are produced by applying a postforming such as skiving to a cylinder obtained by one of the above-mentioned methods. With this method, however, the processing cost is high and continuous production of thin sheets on a large scale is quite difficult.
The difficulty in processing the UHMWPE arises from the fact that it has quite a high melt viscosity and does not have good melt flow properties, and therefore, the conventionally employed blown-film extrusion method and T-die method in which the pressure drop is large are not suitable.
In consequence, it is strongly desired on an industrial scale to manufacture the lubricating parts directly from the crude powder of the UHMWPE, if possible.
In general, if an attempt is made to thermally mold the lubricating parts from UHMWPE crude powder, the latter will be concentrated on the bottom of a mold, and if compression molding is carried out in the state of such an inclined distribution, the resulting article will have a bottom portion locally thickened.
In particular, it is very difficult or actually impossible that a thin-wall article having a non-planar form such as cylindrical, hemispherical, spherical or corugate shape is molded with a high thickness accuracy by the compression molding. The reason is as follows: In order to mold the article having an accurate thickness, it is necessary that the UHMWPE crude powder be distributed in a predetermined thickness in the mold, but the UHMWPE crude powder which freely flows at room temperature cannot be set, against the gravity, in predetermined thickness in the mold.
Moreover, the molded article having an arbitrary hollow shape such as a pipe, an elbow, a tee, a vessel, or a profile cannot be easily manufactured from the crude UHMWPE powder.
It is also difficult to manufacture composite molded articles from the UHMWPE material.
For example, for the purpose of giving slipping properties and wear resistance to the surface of a rubber, while rebound or non-rebound properties of the rubber are maintained, it is desirable to superpose a thin UHMWPE layer having a thickness of about 0.5 mm on the rubber. However, when the usual compression molding method is employed, the UHMWPE crude powder cannot be set uniformly and thinly on the surface of the rubber. Thus, the UHMWPE layer is first molded and then skived to prepare a desired thin sheet. Afterward, the thus prepared thin sheet is superposed on the rubber. It is natural, therefore, that the article manufactured in this way is expensive.
For improvement of poor stiffness of the UHMWPE layer, it is desirable to superpose the said layer on a metallic plate, a thermosetting resin, a thermoplastic resin, a cloth or the like having a less elongation. However, a composite sheet having the thin UHMWPE layer cannot be made from the UHMWPE crude powder by the compression molding method because the UHMWPE crude powder cannot be set in the mold with an accurate thickness. The UHMWPE cannot be, therefore, applied to a fender or a door panel for an automobile for which thinness, high stiffness and high impact resistance are necessary.